Optimization of Biomass and Lipid Production in Heterotrophic Microalgal Cultures.

摘要

Microalgae are a promising source of biofuels and other valuable chemicals. The low cell density and slow growth rate that have traditionally characterized microalgal cultures, however, have resulted in a reduced economical feasibility. To develop a sustainable microalgal process it is required to increase culture productivity, maximize production yield, and reduce production costs. To achieve these goals it is necessary to improve the current understanding of the dynamic behaviour of microalgal cultures.;In this thesis, growth and oil production rates in heterotrophic cultures of Auxenochlorella protothecoides were evaluated as a function of the carbon and nitrogen source concentration. It was found that nitrogen plays a major role in controlling the productivity of microalgae. It was also shown that there exists a nitrogen source concentration at which biomass and oil production can be maximized. A mathematical model that describes the effect of nitrogen and carbon source on growth and oil production was proposed, considering the uncoupling between nitrogen uptake and growth, the possibility of luxurious uptake of nitrogen, and the time-delayed inhibitory effects caused by the transient spike in the intracellular nitrogen concentration.;Using a non-linear model-based optimization approach, biomass and oil productivities were substantially increased. The use of an adaptive model predictive control strategy resulted in a 10-fold increase in the average biomass productivity and a 16-fold increase in the maximum productivity compared to batch experiments. The final cell density in the optimized culture was 144 g/L (dry weight), with 49.4%w/w oil content. The maximum lipid productivity was 20.2 g/Ld, achieved during the exponential growth phase at an average cell density of 86 g/L. The lipid productivity in the optimized microalgal culture was higher than any previously reported productivity value for other oleaginous microorganisms.;Application of the adaptive optimization strategy to a two-stage glycerol/ glucose culture resulted in an increased production yield (glucose to oil), from 0.267 g/g in the optimized single-stage culture to 0.347 g/g in the twostage culture. The increased yield and productivity of the optimized cultures resulted in a largely improved economic feasibility.;Composition analysis of the algal oil produced in the optimized cultures shows that the oil has a high quality as biodiesel precursor, in terms of the expected cetane number, iodine value, and cold filter plug point temperature. The higher productivity and excellent lipid profile of the optimized microalgal culture make A. protothecoides an exceptional source for biodiesel production and a potential source of single cell oil for other applications.